Vehicles typically have at least one seat for an occupant to sit in while the vehicle is in use. Vehicle seats may include a base adjustably mounted to a floor of the vehicle and a backrest adjustably mounted to the base, although other seat designs have been used, such as a continuous seating base and backrest design, or multiple seating bases with multiple backrest designs, or the base or backrest of the seat may be provided by a vehicle structure, or any combination of the above. The base and backrest, whether they are separate components or a continuous unit, typically have a structural perimeter frame with a suspension system disposed therein. The suspension system is used to support an occupant and to provide comfort while they are seated. Suspension systems typically consist of a combination of box, coil springs, or a wire-like mesh suspension system and foam. The suspension systems typically span between at least one pair of opposing structural members of the perimeter frame to facilitate meeting occupant position and comfort requirements.
The base and backrest function to provide a reaction surface for occupant mass and provide occupant support. Base and backrest suspension system stiffness is a fixed value and is dependent upon the spring/foam stiffness combination of the suspension system. Some systems may offer a way for the occupant to vary the stiffness of the seat for comfort, but once the stiffness level is chosen, the stiffness remains a fixed value during use.
Seatbelts are often used to hold an occupant to the sear. During vehicle acceleration, the mass and acceleration of the occupant in the seat may cause the seat and/or backrest to compress such that space between the seatbelt and the seat becomes greater, which may in turn allow the occupant to slide out from under the seatbelt. The term acceleration, as used here, also includes deceleration. The sliding out from under a seatbelt caused by seat compression may be referred to as submarining. Five and six point harness seatbelts help prevent submarining by using crotch straps, but most vehicles are not equipped with five and six point harness systems, as they are perceived as being uncomfortable and inconvenient for everyday use.
A seat may be provided with a pronounced geometry in specific locations or a stiffer base and backrest to increase occupant support and seatbelt restraint during acceleration. However, greater pronounced geometries and stiffness tend to decrease seat comfort, as softer seats tend to increase comfort.